DC/DC converter for low output voltages

ABSTRACT

With direct current/direct current converters of modern construction, which can be operated at frequencies as high as 500 kHz, the number of secondary windings that are necessary decreases with simultaneously high currents, so the necessary voltage can be generated by a single winding, or even by less than one winding. A push-pull converter exhibits, for example, a transformer with a center limb (19) and two outer limbs, and that contains on the secondary side two coils (29, 30), each of which consists of a single, complete winding with one center tap (B, E) each. These coils are placed around the center limb (19) in such a way that the two center taps (B, E) come to be located opposite each other in two openings (18) between the outer limbs. In the same opening (18) as the center limb (E) of the one coil (30), the terminals (A, C) of the other coil (29) are also located, and in the other opening (18), in which the center tap (B) of the other coil (29) is located, the terminals (D, F) of the one coil (30) are also located.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to DC/DC converters and, moreparticularly, to a DC/DC converter having a transformer for providinglow output voltages and high currents.

2. History of the Related Art

Voltage regulators, both those that work according to theforward-current converter principle as well as those that work accordingto the fly-back converter principle, have become very well known in thelast several years, for example, from O. Kilgenstein, Schaltnetzteile inder Praxis, Wuerzburg, 1992, to mention one good overview. It is alsotrue that in the last several years the components used in these deviceshave also been continuously improved, so that it has become possible toincrease the working frequencies from several kiloherz in the beginning,to nearly 500 kHz today. Along with that, however, the total number oftransformer windings in such switching regulators also decreases aswell. In addition to that, increasingly lower output voltages are neededtoday. Along with the 5 volts that used to be common, output voltages ofaround 3 volts are now starting to appear, or even 2 volts. Since in thedimensioning of the transformers for 5-volt switching regulators,engineers are already getting down to a single secondary coil at times,voltages that are smaller yet, with currents that are often high, whichcan no longer be realized economically.

The present invention provides the creation of a direct current/directcurrent transformer for small output voltages and high currents, whileregaining latitude for development despite the mentioned limitations,which result from the nature of the matter.

SUMMARY OF THE INVENTION

The present invention relates to a DC/DC converter with a primarysection which chops the primary voltage, a transformer having a primarycoil and at least one secondary coil, a secondary section that issupplied by the transformer and that has a storage choke and a storagecapacitor that is charged to the secondary voltage.

More particularly, one aspect of the present invention relates to theDC/DC converter described above wherein the secondary coil of thetransformer consists of several fractional winding segments throughwhich current flows symmetrically.

In another aspect, the present invention relates to the DC/DC converterdescribed above wherein the secondary coil of the transformer consistsof n segments that are of the length of an integral fraction of oneentire winding, each of which segments thus exhibits the length 1/n ofone entire winding, and n represents a small integer.

In another aspect, the present invention relates to the DC/DC converterdescribed above wherein the transformer includes two secondary coils,each of which consists of a half-winding.

In another aspect, the present invention relates to the DC/DC converterdescribed above wherein the secondary coil is divided into n sectionswhereby each section encompasses m/n of a winding, the n sections areconnected in parallel and m and n are small integers.

In another aspect, the present invention relates to the DC/DC converterdescribed above wherein the length of one segment of the secondary coilis an improper fraction of one complete winding and thus exhibits theform m/n×length, where m≧n, and n number of such segments are presentand m and n are small integers.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention will become more apparentwith reference to the following detailed description of a presentlypreferred embodiment thereof in connection with the accompanyingdrawings, wherein like reference numerals have been applied to likeelements, in which:

FIG. 1 is a schematic diagram showing a known forward-current converter;

FIG. 2 is a sectional view of a known transformer pot core;

FIG. 3 is a top view of a pot core;

FIG. 4 is a top view of a so-called X-core;

FIG. 5 is a top view of a first embodiment of a transformer inaccordance with the present invention;

FIG. 6 is a schematic diagram showing a forward-current converter thatmakes use of the inventive transformer in FIG. 5;

FIG. 7 is a variation of FIG. 6;

FIG. 8 is a schematic diagram showing a push-pull converter with atransformer in accordance with the present invention;

FIG. 9 is a second embodiment of a transformer in accordance with thepresent invention;

FIG. 10 is a third embodiment of a transformer in accordance with thepresent invention.

DETAILED DESCRIPTION

FIG. 1 shows, as one example of many, a known forward-current converter.A current source, shown as battery 1 with a first voltage U₁, suppliesvia an electronic switch 2, the primary coil 4 of a transformer 3. Anadditional coil that is located on the primary side of the transformer3, the demagnetizing coil 6, is supplied by means of a diode 7. On thesecondary side, the transformer 3 exhibits a secondary coil 5, theforward current of which charges a storage capacitor 11 via a diode 8and a storage choke 10. The other side of the secondary coil 5 isconnected on one hand with the cathode of a recovery diode 9, and on theother, with the other terminal of the storage capacitor 11. The recoverydiode 9 conducts during the other half of the cycle. The voltage U₂through the storage capacitor 11 is simultaneously present at twooutputs, 12 (positive) and 13 (negative). An external load is identifiedby the number 14.

The transformer identified by the number 3, likewise state of the art,is shown in FIGS. 2 and 3 in a sectional view and in a top view. Twohalves 15 and 16 that are in principle configured identically, togetherform a so-called pot core. By the joining together of the two halves 15and 16 into a complete whole, there is created a ring-shaped cavity 17that receives the coils, which are not shown here. The ends of the coilsare led out through two openings 18. In the middle of the ring-shapedcavity 17 there is found, identified by the number 19, the center limbof the transformer, which carries the entire magnetic flux generated bythe primary coil 4; the return takes place by means of the outer limbs,which are identified by the number 20.

In FIG. 4, a variation in form with respect to FIGS. 2 and 3 is shown ina top view of the lower half 16 only. The difference versus the pot corein FIGS. 2 and 3 consists in the fact that four openings 18 are present.Pot cores of the type shown in FIGS. 2 and 3 are, in terms of magnetictopology, E-cores, in conjunction with which there is a plurality offorms in use with respect to the configuration of the parts 20 and theopenings 18. The designs that are shown cause the magnetic flux that isgenerated in the center limb 19 to be divided into several partialfluxes, each of which comprises one half or one fourth of the totalflux. It is on this fact that the configuration in accordance with theinvention of the secondary coil 5 per FIG. 1 is based, as it is nowshown in FIGS. 9 and 10.

FIG. 5 shows a pot core in accordance with FIG. 3, the openings 18 ofwhich are wide enough so that there is space for four terminals a, b, cand d from a secondary coil that is made as two half-windings 21 and 22.The following relationships now hold true for the magnetic flux that isgenerated in the core 19 and is returned in the outer limbs 23 and 24(indicated by the vector symbols x and ⊙):

    Φ=(B.sub.K ·A.sub.K).sub.19 =(B.sub.Z ·A.sub.Z).sub.23 +(B.sub.Z ·A.sub.Z).sub.24

where

Φ=magnetic flux

B_(K) =magnetic field in the center limb

B_(Z) =magnetic field in outer limb

A_(K) =surface vector of the center limb

A_(Z) =surface vectors of the outer limbs

and each of the indices 19, 23 and 24 identifies the component.

For reasons of symmetry, it also holds true that

    (B.sub.Z ·A.sub.Z).sub.23 =(B.sub.Z ·A.sub.Z).sub.24

Thus, for example, the half-coil 22 encompasses half of the flux,regardless of whether the current path I encompasses the branch 24(dotted line) or the center limb 19 and outer limb 23 (dotted/dashedline): If it encompasses the outer limb 24, then

    Φ.sub.cd =Φ/2,

if it encompasses the center limb 19 and the outer limb 23, then

    Φ.sub.cd =-(-Φ+Φ/2)=Φ/2,

because of the reversal of polarity of the vector of the surfaceencompassed by the current path. ##EQU1## the induced voltage isstrictly proportional to the encompassed surface: A half-winding thatencompasses half of the flux thus generates half of the voltage of acomplete winding that encompasses the entire flux.

This can be generally extended to any desired fraction of the flux andto any desired partial windings. FIG. 6 shows the secondary section ofthe forward-current converter in accordance with FIG. 1, with asecondary coil divided according to FIG. 5 into two half-windings 21 and22. According to the diagram in FIG. 6, they are connected in parallel;the designations a, b, c, and d of the ends of the half-windings 21 and22 have been carried over from FIG. 5, which is also indicated by thedots next to b and d.

Of course, the half-windings 21 and 22 can be lengthened by any desiredmultiple of 1/2, and thus exhibit 1, 11/2, 2, 21/2 windings. Thecorresponding also holds true for quarter-windings, for example, whichcan be inserted into a pot core in accordance with FIG. 4. Fourquarter-windings then bring about a corresponding modification of FIG. 6as a consequence as well. Four secondary coils are connected inparallel.

FIG. 7 is an additional representation of a forward-flow converter withtwo half-windings. Here, the diodes 8 and 9 of FIGS. 1 and 6 have beenreplaced by electrical switches, for example, MOSFETs 25, 26, 27 and 28.The corresponding drives and the primary section have been left off. Incorrespondence with the doubling in number of the switching elements,the storage choke 10 is also made with two parallel coils, as a resultof which the high currents are taken into account.

The subdivision of the secondary coil into fractional windings inaccordance with the invention is particularly advantageous for push-pullarrangements of all kinds. FIG. 8 shows an embodiment of a push-pullconverter with known primary-side wiring. On the secondary side, thetransformer 3 consists of two coils 29 and 30 with one winding each,whereby both of them exhibit one center tap (B, E) each. The two centertaps B and E are permanently connected directly to the input of thestorage choke 10, while the ends A and C and the ends D and F of thecoils 29 and 30 are alternately connected to the potential of thenegative output 13 via four electronic switches 31, 32, 33 and 34, forexample, MOSFETs.

FIG. 9 shows the geometry of the secondary side of the transformer 3.The two coils 29 and 30 are placed around the center limb 19 in such away that the center taps B and E come to be located opposite each other,one of them in each of the two openings 18. The terminals A and C andterminals D and F are located in the same opening 18 as the center tapsE and B respectively. Instead of being concentric with one another, thecoils 29 and 30, using the identical form and size, can also lieinsulated from one another and on top of one another.

FIG. 10 shows a variation of FIG. 9. Here, the secondary coils of thetransformer 3 exhibit only one half-winding 35 and 36 each, with onecenter tap B and E each, so that only one quarter-winding each extendsfrom the center taps B and E to the terminals A and C and terminals Dand F respectively. Because of the load symmetry of the transformercore, each half-winding 35 and 36 is made double, and the half-windingswith the same identification number are connected in parallel.

One variation of a transformer in accordance with the invention that isnot shown exhibits three openings 18 that are displaced 120° from oneanother; each of the two windings extends over an arc of 240°, and againexhibits a center tap. They are placed into the ring-shaped cavity 17displaced by 120°.

The present invention is particularly advantageous whenever severaldifferent voltages are to be generated in a single direct current/directcurrent converter. For example, if 5 volts, 3.3 volts, and 2.5 volts areplanned as regulated output voltages, and if the raw voltage for the5-volt output can already be generated by means of a secondary coil withone single winding, then using appropriate pot cores, secondary coils of2/3 and 1/2 winding respectively are added for the other two voltages.

We claim:
 1. In a direct current/direct current converter for lowvoltages, both in accordance with the forward-current converterprinciple, as well as with the fly-back converter principle and thepush-pull converter principle, with a primary section that chops theprimary voltage, a transformer (3) that is supplied by the choppedprimary voltage (U₁) and that has a primary coil (4) and at least onesecondary coil (21, 22, 29, 30), a secondary section that is supplied bythe transformer (3) and that has a storage choke (10) and a storagecapacitor (11) that is charged to the secondary voltage (U₀), wherebythe transformer (3) exhibits a center limb (19) which carries themagnetic flux Φ generated by the primary coil, and exhibits at least twoouter limbs (23, 24), to which the cited magnetic flux Φ is distributedin equal portions, and the outer limbs (23, 24) are separated byopenings (18), the improvement comprising: the secondary coil (21, 22,29, 30) consists of several fractional winding segments through whichcurrent flows symmetrically.
 2. The direct current/direct currentconverter as set forth in claim 1 wherein the secondary coil (21, 22,29, 30) consists of n segments that are of the length of an integralfraction of one entire winding, each of which segments thus exhibits thelength 1/n of one entire winding, and n represents a small integer. 3.The direct current/direct current converter as set forth in claim 2wherein two secondary coils (21, 22) are present, each of which consistsof a half-winding.
 4. The direct current/direct current converter as setforth in claim 2 wherein two secondary coils (29, 30) are present, eachof which consists of a whole winding that exhibits one center tap (B, E)each, and that the length of each coil from each end to the center tap(B, E) consists of a half-winding.
 5. The direct current/direct currentconverter as set forth in claim 2 wherein the secondary coil (21, 22,29, 30) is divided into n sections, whereby each section encompasses m/nof a winding, the n sections are connected in parallel, and m and n aresmall integers.
 6. The direct current/direct current converter as setforth in claim 1 wherein the length of one segment of the secondary coil(21, 22, 29, 30) is an improper fraction of one complete winding andthus exhibits the form m/n×length, where m≧n, and n number of suchsegments are present, and m and n are small integers.
 7. The directcurrent/direct current converter as set forth in claim 6 wherein thesecondary coil (21, 22, 29, 30) is divided into n sections of the lengthm/n, and all n sections are connected in parallel, and m and n are smallintegers.
 8. The direct current/direct current converter as set forth inclaim 5 wherein several secondary coils (21, 22, 29, 30) are present,which supply several outputs of the direct current/direct currentconverter and which have varying lengths.